1. Field of the Invention
The present invention relates to a semiconductor device with a heat sink and a method of producing the heat sink, and particularly to a semiconductor device with a heat sink and a method of producing the heat sink, in which the heat radiating surface of the heat sink of the convex type as a heat radiator is exposed to improve the heat radiation property and reliability of the device.
2. Description of the Prior Art
Recently, with the increase of power consumption of a device such as a high integrated CMOS LSI, a demand for a low-cost and low-heat-resistant plastic package has become stronger and stronger. As a countermeasure to this, from the viewpoint of material, a method of making a lead frame or sealing resin have a high heat conductivity has been proposed, and from the viewpoint of structure, a method of changing the design of a lead frame or adding a heat sink, that is, a heat radiator has been inspected or has been put into practice.
Though the description of advantages disadvantages and features of these methods is given away to special books, the method of adding a heat sink to reduce the heat resistance of a package can be regarded as the most orthodox countermeasure for LSIs of power consumption up to about 2 W per chip in view of the present situation including the near future one.
FIG. 14 is a main portion sectional view schematically illustrating a package structure of a conventional semiconductor device with a heat sink of a partially exposed heat radiator type. In FIG. 14, the reference numeral 1 represents each of leads of a lead frame. A number of leads 1 are arranged so that a quardrangular space portion is formed in the center surrounded by the forward end portions of the respective leads 1. A heat sink 4 constituted by a convex body of high heat-conductive metal such as oxygen-free copper (the structure of which will be described later more in detail) is bonded and fixed at a circumferential portion of its bottom, onto the leads 1 through an insulating plate 2 such as a polyimide plate. A semiconductor chip (semiconductor element) 3 is attached onto the bottom center portion of the heat sink 4 at a position within the above-mentioned space portion by a bonding agent or the like. Bonding pads (not shown) of the semiconductor chip 3 are connected to the corresponding leads 1 through wires 5 respectively.
In the above-mentioned manner, the semiconductor chip 3 connected to the leads 1 and mounted on the position within the above-mentioned space portion is sealed with sealing plastic such as epoxy resin except part (for example, outside portions) of the respective leads 1 and part (for example, the top surface of a projecting portion) of the heat sink 4, so as to form a package 6. The respective portions of the leads 1 projecting out of the package 6 are bent into predetermined shapes so as to form terminals of the device. Thus, the formation of a semiconductor device with a heat sink is completed.
With the above-mentioned configuration, heat generated from the semiconductor chip 3 in use reaches the top surface of the above-mentioned projecting portion of the heat sink 4 effectively through the body of the heat sink 4 which is a low heat-resistant (high heat-conductive) body. Thus, the heat is radiated by air cooling to the outside from the top surface of the projecting portion of the heat sink 4.
FIG. 15 is an enlarged sectional view of the heat sink illustrated in FIG. 14. A conventional convex heat sink 4 is produced by cutting and rolling in combination. Therefore, even if production is made so that a perfect convex body having a convex portion (that is, projecting portion) which has vertical side surfaces should be obtained as the convex body illustrated in FIG. 14, the body produced in practice has rounded or curved corner portions or irregular cut burrs because of working strain or working defect in cutting or rolling respectively as shown in portions A and B in FIG. 15. Although the cutting burrs can be removed in the following process, it is general to use the heat sink with the curved corners left as they are.
In such a conventional semiconductor device with a heat sink and a conventional method of producing such a heat sink, particularly because the heat sink is produced by machining, it is difficult to obtain proper flatness and parallelism of a top surface of a projecting portion of the heat sink, so that the edge corner surfaces of the projecting portion of the heat sink are curved. Therefore, the top surface of the projecting portion near these edge corner surfaces is inclined like a gentle descent, and in the case where the curvature of the curved surface of an upper portion of a side portion is too large, the above-mentioned inclined portion is involved in resin at the time of sealing with the resin so that the resin is put thereon. Accordingly, there has been a problem that separation occurs at the boundary between the sealing resin and the heat sink to thereby make the reliability of a semiconductor device of this type deteriorate.
In addition, also from the viewpoint of macrostructure, a conventional heat sink has a problem that the adhesive property of the heat sink with the sealing resin becomes low so that troubles caused by this reduction in adhesion reduce the reliability of a semiconductor device.
Further, there have been problems that not only it is difficult for a bonding machine to automatically recognize leads of a lead frame in a because the surface of a heat sink has brightness in producing a device, particularly at the time of wire bonding, but also the quality of bonding is not stable because an insulating plate is elastic enough so that it is strained.